Papermakers would benefit from a simple, effective, starch-based, cellulose-reactive surface-applied sizing agent system that (i) imparts useful sizing properties to fibrous substrates and (ii) reduces or eliminates the need to use sizing agents at the wet end of a papermaking process. Unfortunately, known methods and compositions have prevented papermakers from achieving this goal.
It is well known that the property of sizing, as applied to paper, refers to a fibrous substrate's ability to resist wetting or penetration of a liquid into a paper sheet. Aqueous dispersions of alkenylsuccinic anhydride (ASA) cellulose-reactive sizing agent have been widely used in the paper and board making industry for many years, for sizing a wide variety of grades which include printing and writing grades and bleached and unbleached board grades. Cellulose-reactive alkenylsuccinic anhydride emulsions impart hydrophobic properties to the paper and board products.
Chemicals used to achieve sizing properties are known as either internal sizes or surface sizes. Internal sizes can be either rosin-based or synthetic sizes such as alkenylsuccinic anhydride, or other materials. Internal sizes are added to the paper pulp prior to sheet formation. Surface sizes are sizing agents that are added after the paper sheet has formed, most generally at the size press, although spraying applications may also be used.
Alkenylsuccinic anhydride sizing agent is ordinarily applied by dispersing it in a cationic or amphoteric hydrophilic substance such as a starch or a polymer. The starch or polymer-dispersed alkenylsuccinic anhydride sizing emulsions have been added to the pulp slurry before the formation of a paper web. This type of addition of alkenylsuccinic anhydride sizing emulsions to the papermaking system is commonly called wet-end addition or internal addition of alkenylsuccinic anhydride.
Unfortunately, the addition of alkenylsuccinic anhydride to the wet end of the papermaking machine has several disadvantages. Internally added alkenylsuccinic anhydride emulsions are never totally retained on the fiber. The portion that is not retained is free to react with water or other components of the papermaking system and can form deposits at the wet-end of the paper machine, or can then be carried to the press or drying sections of the paper machine and form paper or board defects. Further, internal addition of alkenylsuccinic anhydride emulsions has the potential for interacting with other wet-end additives, such as brightening agents, defoamers or dispersants, biocides, dyes, strength agents, etc.
Further, increases in filler addition, such as calcium carbonate filler at the wet-end of the paper making system have led to an increase in size demand as well. Filler particles have a relatively high surface area as compared to cellulose fiber and readily adsorb internally added sizing agents. Alkenylsuccinic anhydride, which is adsorbed onto calcium carbonate filler particles, leads to a less efficient sizing, requiring higher doses as compared to treatment of unfilled paper webs sized with cellulose reacted alkenylsuccinic anhydride sizing agent.
Efforts to develop compositions and methods that surface treat fibrous substrates have failed to produce a simple, effective starch-based system that imparts useful sizing properties to a fibrous substrate and that reduces or eliminates the need to use sizing agents at the wet end of a papermaking process. For example, conventional surface sizes, styrene acrylate emulsions, styrene acrylics, styrene maleic anhydrides, polyurethanes and the like require an internal size to be efficient.
U.S. Pat. No. 6,162,328 discloses a method for sizing paper that adds a sizing composition containing mixtures of cellulose-reactive and cellulose non-reactive size dispersions to the surface of the paper. The cellulose non-reactive sizes are polymeric materials such as copolymers of styrene or substituted styrenes with vinyl monomers containing carboxyl groups. Cellulose-reactive sizes include sizes such as ketene dimers and multimers, alkenylsuccinic anhydrides, organic epoxides, acyl halides, fatty acid anhydrides from fatty acids and organic isocyanates. The starch may be of any type, including but not limited to oxidized, ethylated, cationic and pearl starch, and is preferably used in aqueous solution. The cellulose-reactive size dispersions and non-reactive size dispersions may be added with a solution of starch or starch derivative before being applied to the paper.
U.S. Pat. No. 6,162,328 requires the combination of at least one cellulose-reactive size and at least one cellulose non-reactive size. This combination allows one to add alkenylsuccinic anhydride or alkyl ketene dimer to the size press by balancing properties of both types. The requirement that combinations of polymeric materials be used makes the composition more expensive and complicated as compared to single sizing component addition. Further, it does not include any criticality in the ratio of starch to the cellulose-reactive size.
U.S. Pat. No. 4,872,951 discloses blends of alkenylsuccinic anhydride-treated and cationic starches for use as external sizes of paper and paperboard products. The blends contain 30-90% (by wt.) of the alkenylsuccinic anhydride-treated starch, which is a monoester of the starch and an alkyl- or alkenylsuccinate and 10-70% (by wt.) cationic starch. The invention requires a reaction product of starch with alkenylsuccinic anhydride combined with cationic starch, which is added to the surface of the paper. Manufacturing this reaction product is an additional process step. In addition, the document's emphasis on cationic starches does not teach how non-ionic and anionic starches could be used in emulsions to effectively deliver alkenylsuccinic anhydride to a fibrous substrate and impart useful sizing properties.
WO 02/08514 describes the preparation of a sizing emulsion that contains a sizing agent, and an inorganic particulate emulsifying agent capable of forming an emulsion and water. The sizing agent can be 2-oxetanone dimer or multimer, alkenylsuccinic anhydride, rosin or carbamoyl chloride. The inorganic particulate emulsifying agent is selected from clay, silica, zeolite, mica, calcium carbonate, phosphate or sulfate; aluminum oxide, hydroxide, phosphate or silicate; magnesium phosphate or silicate; polyaluminum chloride, phosphate or silicate and ferrous or ferric phosphate, silicate or oxide. According to the patent, the addition of the inorganic particulate emulsifying agent allows one to add alkenylsuccinic anhydride to the size press. Example 28, a comparative example, discloses that a conventionally prepared alkenylsuccinic anhydride “emulsion comprising surfactant and starch does not work in the size press . . . ”
For the foregoing reasons, there is a need to develop a method that avoids the deposits that are often associated with internally added alkenylsuccinic anhydride sizing agents or ketene dimer sizing agents to the papermaking processes.
For the foregoing reasons, there is a need to develop sizing compositions that provide for nearly 100% retention onto the surface of the preformed fiber web.
For the foregoing reasons, there is a need to develop a surface applied alkenylsuccinic anhydride sizing system that is more efficient than an internally applied alkenylsuccinic anhydride sizing system.
For the foregoing reasons, there is a need to develop sizing compositions that impart useful sizing properties to a fibrous substrate when the sizing compositiontreats a fibrous substrate.